This invention relates generally to wafer slicing and, particularly, to methods of slicing a semiconductor wafer for improved quality control in laser marking the wafer.
Most processes for fabricating semiconductor electronic components start with monocrystalline, or single crystal, silicon in the form of wafers. Semiconductor wafers are produced by thinly slicing a single crystal silicon ingot into individual wafers with a cutting apparatus, such as a wire saw. In general, the wire saw uses a wire mounted on rollers for cutting the ingot. The drive mechanism of the wire saw moves the wire back and forth in a lengthwise direction around the rollers at an average speed of, for example, 10 to 15 meters per second. Commonly assigned U.S. Pat. No. 5,735,258, the entire disclosure of which is incorporated herein by reference, discloses a wire saw apparatus for slicing silicon wafers.
The wafer slicing process typically produces undesirable wafer characteristics such as thickness variations, warp, saw marks and kerf loss. These undesirable characteristics usually can be reduced to a satisfactory level or eliminated by presently available post-slicing processing operations. For example, each as-cut wafer undergoes a number of processing operations to shape it, reduce its thickness, remove damage caused by the slicing operation, and to create a highly reflective surface. In addition, the wafers typically undergo various inspections as part of post-slicing processing.
Wafer producers often use identification marks on the silicon wafers to track them through the various wafering processes. In this manner, different marks can be used to indicate different wafer characteristics, identify the source of defective wafers or otherwise trace the origin of a particular wafer or lot of wafers. For example, a series of laser-scribed dots (also referred to as hard marking) may be used to form an identification number on the front surface of a wafer. Lumonics sells a number of suitable dot matrix machines under the trademark WaferMark.RTM. for hard marking identification marks on silicon wafers with a laser.
Those skilled in the art recognize that hard marking as-cut wafers provides cost benefits over, for example, soft marking polished wafers. In addition, marking wafers relatively early in the process, before they undergo further operations, permits more accurate and complete tracking of the wafers through the various stages of wafer processing. However, a problem associated with hard laser marking as-cut wafers is the inability to accurately control the dot depth and diameter of the marks on the final polished wafer. This is because of the thickness variations and wire marks that are typically present with wafers sliced with a wire saw (conventional as-cut wafers have a thickness distribution with a standard deviation of approximately 3 .mu.m and a thickness variation around the wafer edge of approximately 8 .mu.m).
FIG. 1 is a fragmentary, cross-sectional view of an as-cut wafer 12 sliced in accordance with the prior art. The back and forth cutting action of the wire saw causes wire marks, indicated generally at reference character 14, on a surface 16 of the wafer 12. The spacing between the wire marks 14 is a function of the back and forth cycle time and the speed of relative motion between the ingot being sliced and the wire. Typically, this spacing is on the order of about 1/3 mm (approximately 387 .mu.m in FIG. 1). For convenience, the spacing between wire marks 14 is indicated by reference character T shown between high spots 20 on the surface 16 of wafer 12. The depth of marks 14 depends not only on the back and forth cycle time of the wire saw but also the size of the abrasive in the wire saw slurry. In FIG. 1, the depth D of wire marks 14 relative to the high spots 20 is about 24 .mu.m.
FIG. 1 also illustrates a series of laser-scribed marks 22, which form part of an identification marking on the surface 16 of wafer 12. Preferably, each mark 22 has a generally cylindrical wall and a generally hemispherical bottom and appears as a dot when viewed from above. Conventional marking techniques include marking the surface 16 of wafer 12 with an identifying code formed in an 8.times.32 grid of dots (often referred to as a T7 marking) (see FIG. 2). In addition, wafer 12 is usually marked with an alphanumeric identifier (often referred to as an M12 marking) (see FIG. 2), which is also made up of a series of laser-scribed marks 22.
As described above, each as-cut wafer 12 undergoes a number of processing operations to shape it, reduce its thickness, remove damage caused by the slicing operation, and to create a highly reflective surface. These post-slicing processes typically involve the removal of up to 50 .mu.m or more of semiconductor material from the surface 16 of wafer 12. A line 24 shows the position of an exemplary final polished surface of wafer 12. As a result, thickness variations in general, and wire marks 14 in particular, make it difficult to meet customer specifications for dot depth and diameter on the final polished wafer when the initial marks are placed on an as-cut surface, such as surface 16. Exemplary laser marking specifications call for dots having a depth of 45 .mu.m.+-.15 .mu.m and a diameter of 85 .mu.m.+-.15 .mu.m. According to conventional laser marking techniques, a laser apparatus scribes marks 22 into the surface 16 of wafer 12 at a depth of about 100 .mu.m. After removal of material in the post-slicing operations, the depth and/or diameter of the laser markings may or may not fall within the customer specifications for finished wafers. FIG. 1 shows that the depths of marks 22 relative to the line 24 may vary significantly from one mark to another.
For these reasons, a method of slicing wafers that reduces thickness variations and wire marks and that permits accurate dot depth and diameter control of hard laser marking identification numbers is desired.